Does the number of antennas matter ?

Traditionally, the majority of 802.11b/g routers had two antennas, Access Points had one, however with older routers it is mostly a design choice by the manufacturer to provide automatic "antenna diversity" feature that monitors the signal from each antenna and automatically switches to the one with the stronger signal. The user usually has no control over this function.

Newer 802.11n and 802.11ac devices implement MIMO technology, and use separate antennas for increasing throughput (combined transmit/receive over multiple antennas). A 2x2 wireless router, for example can transmit/receive two "spatial streams" at the same time.

802.11n devices use 150Mbps spatial streams, and typically combine up to three to achieve 450Mbps maximum theoretical throughput with 3x3 MIMO technology.

As of the fall of 2014, some of the latest AC2400 devices are beginning to implement 4x4 MIMO, combining 4 antennas together for even greater throughput. Theoretically, the newest 802.11ac standard can combine up to eight spatial streams.

Notes:Client devices have to support multiple streams with multiple antennas in order to benefit from the improved throughput that MIMO routers offer.Some routers on the market have 6, even 8 antennas and still only support 3 streams. The rest of the antennas are used for beamforming and diversity, but only up to three antennas are used at a time for the same client connection.

I have two separate USB wireless antennas plugged into my WinXP SP2 system and when one antenna drops the connection the other usually is there to pick it up, it works better than one antenna since my one antenna config started dropping connection all the time.